Search

The frequency of infection, lesion characteristics and anatomical changes in phloem and cambial tissues caused by Armillaria sinapina were studied on inoculated trees of Douglas-fir, western hemlock and western redcedar and compared with results of A. ostoyae inoculations previously reported on the same host species. Similar percentages of inoculations resulting in infection of roots on the three hosts indicate that A. sinapina and A. ostoyae are equally pathogenic. Armillaria ostoyae was more virulent than A. sinapina as demonstrated by fungal exudates from A. ostoyae inoculum blocks, which appeared to cause lesions on roots; the higher frequency at which lignified impervious tissue ( IT) and necrophylactic periderm ( NP) developed in bark and following cambial invasion, compartmentalization; the large proportion of roots that showed no visible host response; the large zones of IT formed under continuous stimulation by A. ostoyae advancing in inner bark; and the high frequency of breaching of NP barriers. Spread of A. sinapina mycelium in host species appeared slower than that of A. ostoyae, particularly in Douglas-fir and western hemlock. In western redcedar, A. ostoyae induced stronger host responses than those following invasion by A. sinapina, which included further expansion of the induced rhytidome response, traumatic phloem resin duct formation and higher numbers of polyphenolic parenchyma comprising its barrier zone. Where damage by A. sinapina ensued, it was always associated with high inoculum potential. The ecology of virulent and less virulent species of Armillaria in natural forests is discussed. [ABSTRACT FROM AUTHOR]

A parallel investigation to characterize host reactions involved in wound repair following abiotic injury and challenge with Armillaria ostoyae in three conifer species (Douglas-fir, western hemlock and western redcedar) was conducted in the southern Interior of British Columbia. In this study, we characterize wound healing in roots following freezing injuries to the bark. Five weeks following wounding, all conifers developed lignified impervious tissue (IT) around killed tissue, and a necrophylactic periderm (NP) formed internally abutting IT, although IT was difficult to discern in western redcedar. Phellogen restoration in the secondary phloem of some western hemlock roots was retarded around large clusters of sclereids. Displacement of phloem fibre cells occurred in western redcedar but did not appear to delay NP formation. In roots exhibiting expansion of necrosis to the vascular cambium, callus tissue was evident at the margin of the wound. Novel host reactions in western redcedar involving induced rhytidome formation and the formation of traumatic phloem resin ducts are described. [ABSTRACT FROM AUTHOR]

Structural responses in the bark and wood were described following penetration by Armillaria ostoyae in the roots of 20- to 30-year-old Douglas-fir, western hemlock and western redcedar trees. Tissue necrosis presumably caused by fungal exudates was commonly observed at inoculum contact. In Douglas-fir and western hemlock, A. ostoyae interfered with the initiation of active defence mechanisms involving the development of a lignified zone of impervious tissue (IT), necrophylactic periderm (NP) formation and compartmentalization of infected woody tissue. Breaching of IT and NP barriers was frequent, particularly around the clusters of sclereid cells in western hemlock. In western redcedar, the IT zone was inconspicuous. Induced rhytidome formation occurred in western redcedar either simultaneously with or after completion of NP development. The formation of this tissue facilitated en masse sloughing of infected tissue from the surface of roots. In western redcedar, traumatic phloem resin ducts formed in tangential bands surrounding the margin of expanded lesions. Effective compartmentalization in western redcedar was achieved by a barrier zone comprised of a higher-than-average number of axial parenchyma that accumulated polyphenolic deposits. A combination of host-mediated defence mechanisms in western redcedar resulted in a significantly higher frequency of effective resistance reactions than in western hemlock or Douglas-fir. [ABSTRACT FROM AUTHOR]

Natural colonization by the root and butt rot causing fungi Heterobasidion spp. on Norway spruce ( Picea abies) stumps following thinning and treatment with the biological control agent Phlebiopsis gigantea was investigated on three sites in southern Sweden 6 years after treatment. The fully treated stumps and control stumps were excavated and sampled to compare the survival of Heterobasidion spp. in the long term. Six years post-treatment, 47 and 11% of untreated and treated stumps, respectively, had Heterobasidion infection. There was no difference in the relative infected area in discs collected from the butt and the roots for the different treatments. Control efficacy was 83% for treated stumps. After 6 years, there were no apparent differences between the remaining infections in treated compared with those in untreated stumps regarding the number of colonies, their size or relative infection area. Although infections, 3 months after treatment with P. gigantea, were significantly fewer and smaller than in untreated stumps, Heterobasidion inoculum can survive for at least 6 years in the stump and, when it does, constitute a risk for neighbouring trees. [ABSTRACT FROM AUTHOR]

Microsatellite markers were used to investigate the population structure of the Ash Dieback pathogen, Hymenoscyphus fraxineus, across a tree species diversity gradient containing common ash, Fraxinus excelsior, in the Hainich National Park in central Germany. Population differentiation was determined at a local spatial scale that may be driven by tree species mixtures. [ABSTRACT FROM AUTHOR]

The occurrence of Chalara fraxinea, the fungus responsible for dieback of European ash ( Fraxinus excelsior), was investigated in the current and previous seed years collected from symptomatic trees in Latvia and Sweden using molecular techniques ( DNA extraction, ITS- PCR, Sanger sequencing). Molecular analysis of seeds revealed the presence of 30 different fungal taxa. Chalara fraxinea was detected in 8.3% of seeds tested from the current year originating from Latvia. The presence of C. fraxinea in seeds of F. excelsior is of great concern to phytosanitary protection authorities in countries outside the current zone of infestation. [ABSTRACT FROM AUTHOR]

Eight European ash (Fraxinus excelsior) genotypes with different known susceptibility to Hymenoscyphus pseudoalbidus were tested against the phytotoxin viridiol and their response described at the microscopic and metabolomic level. All ash genotypes were sensitive to the toxin and necrosis was detectable after 24h. Among the three viridiol concentrations used in the experiment, the lowest concentration (14.7μM) yielded markedly lower mean damage scores compared to those resulting from seedlings tested at higher dosages. The highest damage scores were associated with the susceptible ash clones S-101, S-106 and S-125, but also with resistant clone R-104. Three resistant clones (R-131, R-121, and R-118) had lower mean damage scores compared to susceptible clones. Wilting of leaves was more common 48h after treatment and more pronounced on seedlings with high damage scores. The resulting lesions generally lacked browning of tissue and displayed only surface disruption of cells in direct contact with the toxin. A delay in symptom development was evident on all five resistant clones tested with the two higher concentrations of viridiol. LC-HRMS and MS/MS analyses of ash seedling extracts suggest several secoiridoid compounds as well as compounds related to abscisic acid (ABA) to be produced in response to viridiol. ABA-cysteine and xanthoxin were found at significantly higher concentrations in susceptible clones compared to resistant clones after treatment with viridiol, suggesting a primary role of ABA in response to stress. The results observed in this study suggest that genetic resistance to H. pseudoalbidus among ash genotypes may be explained, in part, by the varied response to phytotoxins produced by the fungus., (Copyright © 2014 Elsevier Ltd. All rights reserved.)